1. Field of the Invention
The present invention relates to a method and an apparatus for preventing cast defects in a continuous casting plant.
2. Description of the Related Arts
As is well known, in continuous casting, molten steel is poured into a mold to form it into a casting having a predetermined cross section, and the casting is then continuously withdrawn from beneath the mold, thereby producing a strand. The continuous casting operation is subjected to a significant influence by the initial solidification of the molten steel within the mold. For instance, if the solidifying shell formed in the mold during the initial solidification stage sticks to the inner surface of mold, or if inclusions are engulfed in the solidifying shell, the solidifying shell will rupture directly beneath the mold and molten steel will flow out. This is known as a Break Out (hereinafter referred to as BO). The BO caused by sticking is referred to herein as "constraint BO", and the BO caused by engulfing inclusions as "engulfing BO".
In addition, if the powder, which is used as a lubricant in the mold, flows nonuniformly on the inner surface of a mold, various defects are formed on the surface of the solidifying shell. The BO and the surface defects formed by the nonuniform flow of powder are hereinafter collectively referred to as cast defects. If a BO occurs, it takes a long time to restore the continuous casting machine to a normal state, and hence productivity is reduced. Also, the formation of surface defects necessitates dressing of the strands, which again incurs a certain amount of plant down-time.
In recent years, there have been rapid advances in an increase in the continuous-casting speed and a direct combination of the continuous casting step with the rolling step. The generation of cast defects in particular is a great hindrance to the implementation of a high speed continuous casting and to the direct rolling, i.e., directly rolling the continuously cast steel sections.
Heretofore, a number of techniques have been proposed to predict or detect cast defects at an early stage, to prevent a BO.
For example, Japanese Unexamined Patent Publication No. 57-152,356 discloses a BO-predicting method in which the average temperature of a mold is detected by a thermocouple embedded within a mold, during a steady casting state, and a BO is predicted by detecting the occurrence of a temperature rise above the average temperature, followed by a fall in the temperature. Japanese Unexamined Patent Publication No. 55-84,259 discloses a method for detecting preliminary phenomena of a BO, in which the temperature values are measured at respective halves of a mold and are compared with one another to obtain the temperature difference therebetween, and this temperature difference is used as an index for detecting a preliminary phenomenon of the BO. Japanese Unexamined Patent Publication No. 57-115960 discloses that an abrupt fall in the temperature below the average temperature is detected by a thermocouple embedded in a mold, and is used for detecting an engulfing of large sized inclusions into the surface of the solidifying shell. Japanese Unexamined Patent Publication No. 57-115962 discloses that the changing rate of the temperature relative to time is detected and then compared with a predetermined range so as to detect anomalies in the solidifying shell.
All of the prior arts disclosed in the above publications involve a fundamental object for predicting a BO. Upon prediction of a BO, usually a lamp, a buzzer, or the like is energized to warn the operators and the operators manually then lower the casting speed based on past experience or in an extreme case, interrupt the casting per se. Accordingly, there is a delay and the time between the time of the BO prediction and the time at which the action by the operator is completed. In this respect, there appears to be room for improvement in the way in which the action of the operator is carried out. As a result, a BO frequently occurs which has been predicted but cannot be timely or effectively prevented. For example, the quality failure in a strand in the form of uneveness or lines due to successive casting, which is caused by a temporary interruption in casting, and a secondary BO, which is caused by malfunctions occurring during the resumption of the casting operation after a casting interruption.
Regarding the controlling of a bath level within the mold during a steady operation period, the casting is carried out to maintain a target level which is measured by a level-detecting device, by which the relationship between the set and measured bath levels is measured during a change thereof, and the pouring nozzle is subjected to feedback control on the basis of the measured relationship, to hold the nozzle-opening at a degree at which the bath level is always maintained within a constant range. When a BO is predicted and the casting speed is to be abruptly decreased, action to abruptly decrease the casting speed is taken, which leads to an abrupt rise in the bath level. Therefore, the above described feedback control cannot prevent problems such as an overflow of molten steel from the mold. The operators must simultaneously implement both the casting speed change and a nozzle-opening control commensurate with the changed casting speed. It is extremely difficult to carry out such simultaneous actions and at the same time maintain the bath level within a predetermined range. A variation in bath level that exceeds a predetermined range is detrimental to the qualities of casting due to the powder engulfing that accompanies such a variation.